An important problem in the biology of mammalian cell growth concerns the production of messenger RNA (mRNA). Since both mRNA and heterogenous nuclear RNA (hnRNA, which include precursor of mRNA), are associated with specific proteins throughout their life history in the cell, our understanding of mRNA metabolism must ultimately be expressed at the ribonucleoprotein level of organization. It is suggested that hnRNA and mRNA bound proteins, which may be associated with specific regions of the RNA molecules, are important determinants in the post- transcriptional regulation of mRNA metabolism. This proposal is aimed at defining the characteristics and specificity of hnRNP and mRNP proteins. The methods include purification of hnRNP and mRNP complexes by affinity chromatography on oligo d(T) cellulose and resolution of their polypeptides by two-dimensional gel electrophoresis. Initial experiments will attempt to characterize the specific proteins from HeLa cell hnRNP and mRNP with respects to their isoelectric points, molecular weight and rate of synthesis. Their possible role in messenger RNA processing will be determined by comparing the rate of turnover in serum deprived resting fibroblasts with that of the hnRNP proteins from normally growing cells after replenishment of serum growth factors. These studies may help elucidate the mechanism of action of cell growth regulatory factors in normally growing cells. Knowledge of the nature of post-transcriptional regulation of gene expression in mammalian cells, induced to proliferate by serum growth factors, would aid our understanding of abnormal growth during malignancy.